1. Technical Field
The present invention relates to an apparatus for detecting a direction of a target, and in particular, to the apparatus equipped with both of a plurality of channels of radio waves to be transmitted and received through antennas and means for detecting the direction of the target using a difference in phase of the radio wave signals received through the channels.
2. Related Art
Among conventional techniques for detecting a direction of a target, one type of technique has been realized by using radio waves. Practically, such detection techniques have been known as a mono-pulse method, a phased array method, a digital beam forming (DBF) method, and others. These detection techniques use a difference in phase of signals received through a plurality of antenna elements.
Specifically, as shown in FIG. 1A, these detection techniques require combinations of antenna elements (called “channels”), which give a difference in paths “ΔL” to radio waves to be transmitted and received to and from a target. A direction “α” of the target, which reflected the radio waves, is thus obtained based on a phase difference ΔL (rad) caused between the signals received through the plural channels.
In such a configuration, however, as shown in FIG. 2, because changes in the phase have periodicity, aliasing in the phase will be caused, thus making it impossible to distinguish Δθ=θ0 (|θ0|<π) from Δθ=θ0±2nπ (n=1, 2, . . . ). Hence if a target is located in an azimuthal angle region A0 (hereinafter referred to as “directional area”) corresponding to a range of −πto +π [rad] in the phase difference Δθ, the direction of the target can be detected correctly. It is however impossible to correctly detect the target if the target exists outside the directional area A0, that is a directional area Am which corresponds to a range of (2m−1)π to +(2m+1)π [rad] (m is an integer other than 0) in the phase difference Δθ. In this case, the target is erroneously detected such that it is located within the directional area A0 (refer to a dashed line in FIG. 3).
To be more specific about FIG. 3, the situation shown therein explains a target M that has actually moved from the directional area A0 to an adjacent directional area A+1 (refer to a solid line). Nevertheless, the target M is detected as if it has moved from an adjacent area A−1 to the directional area A0 (refer to the dashed line). This erroneous detection will occur whenever the target crosses each boundary between the directional areas Am.
In the configuration shown in FIGS. 2 and 3, the wavelength of the radio waves is 3.9 mm (appr. 77 GHz), an interval “d” of arrangement between antenna elements (refer to FIGS. 1A and 1B) is 7.2 mm, the directional area A0 is −16 to +16 degrees, and an angular width of each directional area Am is 32 degrees.
In order to prevent the erroneous detection due to the phase aliasing, it is general that the directivity of the antenna is made narrowed. The directivity is defined as a half angular-value width at which the antenna has a half a maximum gain. Narrowing the directivity prevents reception of the radio waves transmitted from outside the directional area A0. An alternative countermeasure is that, as shown in FIG. 1B, the interval “d” of arrangement of the antenna elements is made narrower, which results in a widened angular width of the directional area A0. FIG. 4 shows a relationship of the phase difference Δθ and the direction “α,” which is gained when the arrangement interval “d” of the antenna elements is made narrower. In this FIG. 4, it can be understood that a target-detectable range (i.e., the directional area A0) is widened, because an amount of rotation of the phase difference Δθ along the direction is made less.
The above techniques for avoiding the erroneous detection are often used by a direction detecting apparatus, of which criteria is based on the phase difference.
Still, Japanese Patent Laid-open publication No. 2000-230974 discloses another erroneous-detection preventing technique. This prevention technique is realized by an apparatus in which two antenna systems are arranged, whose intervals of the antennas are different from each other between the two systems. Based on a difference in the phase between received signals in each antenna system, an arrival direction of the radio waves is sequentially determined, every antenna system, in consideration of the phase aliasing, and when the two-system antenna determine the same direction, the direction is adopted as being a direction which is free from the erroneous detection.
However, this technique is still confronted with some difficulties. One is incompleteness in the foregoing erroneous detection. When the directivity of an antenna is desired to be narrowed, it is very difficult to design an antenna system such that a gain thereof is sharply changed between the inside and the outside of the direction area A0. Thus it is impossible to completely shut down the radio waves coming from the region outside the directional area A0, thus making it difficult to avoid the erroneous detection without fail.
Another difficulty is a lowered performance of the antenna system. When the arrangement interval between two antenna elements is made narrower, an aperture area of each antenna element will become insufficient. This reduces the performance of the antenna system; for example, a maximum detection distance is lowered.
Further, a narrowed arrangement interval between the two antenna elements brings about another problem in the phased-array method and the DBF method. In such methods, it is preferable to make the entire aperture of an antenna array larger, in cases where resolution to identify an arrival direction of radio waves is desired to increase. However, since the arrangement interval between the antenna elements is made narrower, a very large number of antenna elements are required to obtain a certain necessary size of the aperture as the whole antenna array. Thus both of labor for mounting the antenna elements and a manufacturing cost are obliged to be increased.
In the case that the apparatus employs the two antenna systems of which arrangement intervals differ from each other, the construction of the apparatus becomes complicated. In addition, an amount of calculation necessary for possessing reception signals rises remarkably, because the same reception processing should be performed for each antenna system.